xfrm: make xfrm_alg_auth_len() return unsigned int

Key lengths can't be negative.

Comparison with nla_len() is left signed just in case negative value
can sneak in there.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>